In 2017, Tencent developed a still image file format called Tiny Portable Graphics (TPG) based upon the domestic Chinese H265-like video standard AVS2. It apparently is natively supported by their QQ browser: https://intl.cloud.tencent.com/document/product/1045/42363

Even if no mainstream browser supports it and I couldn't find proper format documentation in English, it may still make sense to document browser support. (The AVS site has proposal M4357 for members only.)

There are also other proprietary image formats like SharpP and WXAM, but I think they're not supposed to be send to browsers.